The field of the invention is programmable controllers such as those described in U.S. Pat. Nos. 3,810,118; 3,942,158; 4,165,534; and 4,442,504.
Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the equipment in accordance with a stored program. In programmable controllers such as those disclosed in the above cited patents, for example, the control program is stored in a memory and includes instructions which are read out in rapid sequence and executed to examine the condition of selected sensing devices on the controlled equipment, or to energize or de-energize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices.
The sensing devices and operating devices on the controlled equipment are connected to the programmable controller through input and output circuits which are arranged in separate I/O racks, or I/O modules. Such I/O racks may take a variety of forms such as those disclosed in U.S. Pat. Nos. 4,151,580; 3,992,654; and 4,152,750, and typically, more than one I/O rack is connected to the programmable controller. The I/O racks are connected together by an I/O bus and information is coupled between the I/O racks and the controller processor in accordance with an established protocol. Such protocols are relatively complex and require considerable fault detection and diagnostic hardware and software, such as that described in U.S. Pat. Nos. 4,118,792 and 3,997,879.